1. Field of the Invention
The present invention relates to an apparatus for charging electrical energy accumulation means. It more particularly applies to the production of a power generator intended for a pulse-type electrical supply of a metal vapor laser, e.g. a copper vapor laser.
2. Description of Related Prior Art
It is known that in a metal vapor laser and in particular in a copper vapor laser, a population inversion is obtained by exciting the vapor with high instantaneous power electrical discharges. The energy of these discharges comes from a power pulse generator having electrical energy accumulation means, which has a capacitor or a plurality of capacitors and previously are charged to a high electrical voltage.
The electrical energy accumulation means is associated with switching means by means of which it is discharged into the laser. The switching means can be associated with one or more magnetic compression stages making it possible to decrease the time width of the pulses reaching the laser.
A power pulse generator is described in article (1) by J. K. MITTAL et al entitled "Design And Performance Of A 20 Watt Copper Vapor Laser" and published in J. Phys. E. Sci. Instrum. 21, 1988, pp 388-392, to which reference will be made.
The generator described in said article has a capacitor 2 connected to the positive terminal of a D.C. voltage source 4 across an inductance coil 6 and a diode 8. On the one hand the inductance coil 6 is connected to the positive terminal of the source 4 and on the other to the anode of the diode 8, whose cathode is connected to a terminal of the capacitor 2.
The laser 10 to be supplied is installed between the other terminal of the capacitor 2 and the negative terminal of the source 4. Switching means incorporating a thyratron 12 is installed between the negative terminal of the source 4 and the connection between the capacitor 2 and the diode 8. A capacitor 14, called a picking capacitor and a shunt resistor 16 are fitted between the electrodes of the laser 10.
Other known generators have semiconductor switching devices (e.g. thyristors of the GTO type or transistors of the IGBT type).
In order to assist switching, it is also known to associate saturable magnetic elements (saturable inductance coils) with such semiconductor switching devices. In this connection reference can be made to article (2) by H. J. BAKER et al entitled "An Efficient Laser Pulser Using Ferrite Magnetic Switches" and published in J. Phys. E. Sci. Instrum. 21, 1988, pp 218-224.
Power pulse generators known from the state of the art are subject to instabilities, which lead to fluctuations in the energy of these pulses and therefore to fluctuations of the energy of the light pulses produced by the lasers.
These energy fluctuations are due to the variations in the recharging voltage of the electrical energy accumulation means. The voltage variations are due to fluctuations of the electrical energy absorbed by the laser associated with these accumulation means. This absorbed energy is significantly dependent on the discharge conditions in the vapor of the laser.
The electrical energy reflected at the laser "head" (vapor, enclosure containing the latter and discharge electrodes in the vapor) is in the form of a variable voltage at the terminals of the electrical energy accumulation means and which, during the following charging cycle, leads to the aforementioned fluctuations.
Thus, in the generator described in article (1), the recharging voltage v of the capacitor 2 following a discharge is equal to 2(v1-v0), v1 representing the supply voltage of the capacitor 2 and v0 representing the "initial" voltage (i.e. the voltage after said discharge) at the terminals of the capacitor 2, v0 being to the electrical energy reflected by the laser. It is the voltage v0 and consequently the voltage v which fluctuate.
In the case of generators having semiconductor switching devices associated with saturable magnetic elements, said generators are also subject to instabilities causing a jitter, i.e., a time position instability of the electrical pulses supplied to the laser. This jitter is inadmissible during the placing in series of several lasers operating as amplifiers.
The aim of the present invention is to obviate the instability disadvantages suffered by the known generators for supplying metal vapor lasers and in particular copper vapor lasers, by solving the problem of controlling the electrical energy stored in the accumulation of said generators.
More generally, the invention aims at controlling the charge of the electrical energy accumulation means (e.g. a capacitor or group of capacitors connected in series or in parallel) taking account of a residual charge which said accumulation means may have after discharge.
U.S. Pat. No. 4,405,889 discloses a power supply intended for a repetitive discharge apparatus and FR-A-2 575 010 discloses a supply unit for an electric discharge pulsed laser.